1. Field of the Invention
The present invention relates generally to golf balls containing at least one layer made from an epoxy composition comprising a curing agent. More particularly, the curing agent is a metallic (meth)acrylate such as zinc diacrylate (“ZDA”) or zinc dimethacrylate (“ZDMA”). The composition may be used to form any layer in the golf ball structure such as, for example, a core, intermediate layer, and/or cover. Preferably, the composition is used to form an intermediate or cover layer having optimum hardness properties.
2. Brief Review of the Related Art
Epoxy compositions are used in various applications including industrial adhesives, sealants, films, and paints. Normally, epoxy resins have two components or parts. The two-part epoxy includes a diepoxy prepolymer and a diamine curing agent. The diepoxy prepolymer is a low-molecular weight polymer having terminal epoxy groups. The diepoxy prepolymer is mixed with a diamine curing agent. Upon this mixing step, the parts react and join together to form a cross-linked epoxy polymer having high adhesive strength.
There are different methods for curing epoxy prepolymers. For example, Taylor et al., U.S. Pat. Nos. 7,208,538; 7,396,869; and 7,528,189, the disclosures of which are hereby incorporated by reference, disclose using metallic diacrylate compounds such as zinc diacrylate (“ZDA”), zinc dimethacrylate (“ZDMA”), and mixtures thereof for curing epoxy functional systems. The system is substantially free of conventional curing agents such as polyamides and polyamines. The metallic diacrylate, acting as a curing agent, can provide benefits such as reduced shrinkage, improved clarity of coatings, heat-aging, and shelf-life according to the '538, '869, and '189 patents. There is no disclosure, however, for making golf balls or golf ball subassemblies or golf ball components in the '538, '869, and '189 patents.
The golf industry and consumers have adopted multi-piece solid golf balls for several reasons including ease of manufacturing, material costs, ball properties, and ball playing performance. For example, three-piece solid golf balls containing an inner core, at least one intermediate layer surrounding the core, and an outer cover surrounding the intermediate layer are generally popular among both avid and recreational golfers. The inner core is made of a rubber material such as natural and synthetic rubbers, styrene butadiene, polybutadiene, poly(cis-isoprene), or poly(trans-isoprene). The intermediate and outer cover layers are made of thermoplastic or thermoset polymers such as ionomer resins, polyolefins, polyamides, polyesters, polyurethanes, and polyureas.
Golf balls having intermediate layers and/or outer covers made of ionomer resins are desirable to many consumers because of their playing performance properties. Particularly, ionomer reins can be used to produce “hard” golf balls having good durability, toughness, and impact strength. Golf balls having a hard ionomer cover are generally resistant to wear and tear caused by a golf club repeatedly striking the ball. Moreover, hard golf balls have a higher compression and players tend to achieve good flight distance when using such golf balls. The “hard” balls tend to travel a farther distance than “soft” balls, which is particularly desirable when hitting the ball off the tee. Ionomers and other tough resins can be used to help make the ball harder and more durable.
“Ionomers” generally refer to ionic copolymers of an olefin such as ethylene and a vinyl comonomer having an acid group such as methacrylic, acrylic acid, or maleic acid. The copolymers contain inter-chain ionic bonding as well as covalent bonding. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. Commercially available ionomer resins are used in different industries and include numerous resins sold under the trademarks, Surlyn® (available from DuPont) and Escor® and Iotek® (available from ExxonMobil). Ionomer resins are available in various grades and identified based on the type of base resin, molecular weight, type of metal ion, amount of acid, degree of neutralization, additives, and other properties.
The ionomers can blended with other polymers and ingredients to modify certain properties. For example, the cover material of a golf ball normally contains white pigment or other colored concentrate, or is painted white or other color. A primer coat can be applied to the cover before a logo, symbol, or other mark is ink-printed onto the surface. A clear, protective coating is applied over the printed mark to provide a glossy finish. It is known to prepare cover compositions containing a mixture of ionomer and epoxy group-containing polymers to make the cover more ink-receptive and protect the printed mark.
For example, Fushihara, published US Patent Application 2003/0100385 discloses a multi-piece golf ball containing a rubber core and cover that is formed from a composition comprising a mixture of ionomer and epoxy group-containing polymer. The cover composition contains a white pigment such as titanium dioxide or zinc oxide. No primer coat is needed. A logo or other mark is printed directly on the surface using ink containing a base resin, pigment, and isocyanate compound. The isocyanate compound reacts with the epoxy group of the polymer in the cover material to form a bond. This results in strong adhesion between the printed mark and cover surface.
Fushihara, published US Patent Application 2003/0104880 discloses a multi-piece golf ball having a core and cover made of an ionomer resin. The cover material may be white-pigmented. An epoxy film is applied over the cover material as a primer coating. The epoxy film coating also may be white-pigmented. A logo or other mark may be printed on the epoxy film. Then, a clear polyurethane film is applied over the epoxy coating. The epoxy coating is made by curing an epoxy resin with a polyamide curing agent; and the polyurethane clear coating is made by curing a polyol with an isocyanate curing agent. Because there is good adhesion between the epoxy coating and polyurethane coating, the printed mark does not wear off.
Kamino et al., published US Patent Application 2009/0105012 discloses a multi-piece golf ball. Referring to FIG. 1 in the '012 Publication, the ball contains a rubber core (4), a mid-layer (6) that may be made of a thermoplastic resin such as an ionomer, a reinforcing layer (8) that improves adhesion between the mid-layer, and a cover (10). The reinforcing layer may be made of a two-pack type thermosetting resin such as an epoxy resin that is cured with a polyamide-based curing agent having multiple amine groups and one or more amide groups. The base polymer of the cover material is a thermoplastic polyurethane. The cover is painted with a paint layer (16) that may be a urethane-based resin, epoxy-based resin, or combination thereof. The epoxy paint layer, which has good adhesion to the cover, may be formed by curing an epoxy resin with a polyamide-based curing agent.
As noted above, “hard” golf balls having intermediate and/or cover layers made of ionomer resins have many advantageous properties; however, they also have some drawbacks. For example, players may experience a harder “feel” when their club face makes contact with such golf balls. The player may sense less control over making the shot. The sensation of striking the ball is generally less natural and comfortable with “hard” golf balls versus “soft” golf balls. As opposed to “hard” golf balls, player can better place a spin on “soft” balls and better control their flight pattern. The softer golf ball feels more natural and the player senses more control. Soft golf balls tend to have higher initial spin versus hard golf balls. Balls having higher spin rates are particularly desirable when making approach shots near a golf hole green. Skilled players can place a back-spin on such balls so that they land precisely on a targeted area of the green.
The resiliency or coefficient of restitution (“COR”) of a golf ball (or golf ball sub-component such as a core) means the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize the COR under these conditions. Balls (or cores) with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance. In general, the COR of the ball will increase as the hardness of the ball is increased. The test methods for measuring the COR are described in further detail below.
It would be desirable to develop a cover or intermediate layer material that provides enhanced resiliency along with a soft feel to the golf ball. The material should have good durability, toughness, and impact strength. The material should have high resiliency and COR so that a player can drive the ball long distances. The material, however, should not be so hard and stiff that playing performance properties such as feel, softness, and spin control are sacrificed. One objective of the present invention is to develop a material having an optimum combination of hard and soft properties. The present invention provides a material and the resulting golf ball having these properties as well as other advantageous features and characteristics.